Hot Summers, Calm Seas Are Tipping Point for Tatoosh Island's Red Alga

New research by a UBC zoologist underscores how seemingly gradual changes in climate, combined with more extreme conditions, can bring about dramatic ecological changes.

Reviewing three decades of detailed data, UBC zoologist Christopher Harley found that neither gradual temperature increases nor changes in sea level impacted the distribution of red alga on the shores of Tatoosh Island off Washington State.

However, two isolated summers of abnormally high temperatures and unusually calm seas appear to have had sudden and severe impacts on the island's beds of Mazzaella parksii, a common turf-forming red alga found from California to Alaska. The species has been very slow to recover.

"It's vital to keep an eye on multiple environmental factors when trying to determine how species will react to climate change," notes Harley, assistant professor of Zoology at UBC and lead on the study. "In this case, a combination of rare conditions created a tipping point that wouldn't have been reached if conditions had just involved warm temperatures or just calm seas."

The findings were published last week in the Proceedings of the National Academy of Sciences.

Of particular concern to the study authors is how frequently these combinations of external stressors might reoccur as more gradual, overall warming trends 'lower the bar'.

"If we take predicted temperature increases related to global warming and apply them to our study system, lethal combinations of environmental conditions which previously occurred only about once per decade will begin to happen once every two to four years," says Harley. "That may not give slow-growing species like Mazzaella time to recover."

Also worrying is that these combinations of external stressors are very difficult, if not impossible to predict.

"We could potentially manage some of these situations when the stressors are biological -- anthropogenic nutrient input, or fishing pressure. But we're less optimistic when the conditions involve physical parameters we can't control as easily, like the extreme temperatures, wind velocity and wave height," says Harley, who worked with University of Washington biologist Robert Paine on the study.

Because of its relative isolation and climate, Tatoosh Island is home to large array of seabirds and a diverse community of marine plants and animals. It has become one of the most intensively studied field sites in the world.